1. Field of the Invention
The present invention relates to the field of electrochemical generators, and more particularly that of alkaline batteries with zinc anode.
2. Description of the Related Art
It is known that batteries with an aqueous electrolyte consume water in the course of their operation, and more specifically during the overcharge required for a complete charging of the battery, which produces a decomposition of the water of the electrolyte into hydrogen and oxygen.
There are various ways of managing said consumption of water, in particular:                by limiting the overcharge, at the risk, however, of charging the battery insufficiently;        by using a large excess of electrolyte so as to limit the frequency of the additions of water, a situation which can however be applicable only to stationary sets of batteries, by reason of the excessive loads and volumes that result from it.        
These solutions do not permit the need for periodic interventions by the user, of a more or less frequent nature, to be avoided.
There have been manufactured for a long time maintenance-free, so-called sealed, alkaline batteries, which are nevertheless still fitted with a safety valve opening in the event of excessive internal pressure of the element.
Said batteries employ the principle of the recombination of the decomposition gases of the water. Current examples are the nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) alkaline batteries with cylindrical or prismatic formats which are fitted to portable electric and electronic devices (telephones, computers, . . . ).
The negative electrode there is oversize in capacity terms compared with the positive electrode in a ratio that varies from 1.2 to 1.5 approximately in most cases.
When the positive nickel electrode is completely charged, the voltage of the cell rises, marking the start of the evolution of oxygen, said evolution resulting from the electrochemical oxidation of the water.
During the surcharge of said cathode, the negative electrode continues to be charged.
The oxygen formed at the level of the positive electrode diffuses towards the anode of cadmium or metal hydrides and recombines, either with the metallic cadmium or with the hydrogen adsorbed in the metal hydride. Said diffusion is facilitated by the use of a separator permeable to oxygen and by the use of a reduced quantity of electrolyte.
In an alkaline battery, the reactions observed at the level of the negative electrode are as follows, where M is the metal participating in the reaction:at the positive electrode: 2OH.→H2O+½O2+2e.  [1]at the negative electrode: M+½O2→MO  [2]MO+2e.+H2O→M+2OH.  [3]
French patent 2 788 887 describes the principle of alkaline secondary electrochemical generators with zinc anode, as well as a simple and economic technology of production, which allows high levels of performance to be achieved, in particular in terms of cycle life.
The invention which is the subject of said document relates more particularly to the employment of a zinc negative electrode of the impasted-plasticised type, the active mass of which is composed of a mixture containing at least zinc oxide, a fine conductive ceramic powder and a plastic binder.
According to said technology, the anodic active mass formed as a paste, which is obtained after mixing of the various components and a diluent, is introduced into a three-dimensional collector advantageously consisting of a cross-linked copper foam.
The alkaline batteries with zinc anode, such as nickel-zinc (NiZn) or silver-zinc (AgZn), produced by the assembling of zinc electrodes manufactured as described above, and of nickel or silver cathodes, likewise of the impasted-plasticised type, in a nickel foam support, exhibit an excellent aptitude for cycling, and offer performance levels comparable with or higher than those of the other alkaline secondary generators with nickel positive electrodes. They have, in addition, the advantage of reduced cost and the absence of heavy metals.
The NiZn or AgZn batteries of said technology can operate in “open” mode or in “semi-sealed” or else “sealed” mode.
The general operating principles that apply to NiCd and NiMH alkaline batteries also apply to batteries with zinc anode. Thus, in particular, the zinc negative electrode has surplus capacity compared with the positive electrode.
However, in the case of the nickel-zinc batteries produced according to the technology described in French patent 2 788 887, the surplus capacity of the zinc electrode does not exceed about 20% of the capacity of the nickel electrode, which represents a major difference compared with what is conventionally described in the literature, where the zinc anode generally exhibits a surplus capacity of 250 to 500%, in order to reduce artificially the level of discharge of the anode and to increase its cycle life.
In “open” mode, the end of the charging of the battery is accompanied by a release of oxygen at the positive electrode, and then of hydrogen at the negative electrode when the charging is continued. A periodic addition of water is necessary, corresponding to the quantity of electrolyte decomposed.
In “semi-sealed” mode, the battery is fitted with a valve which opens at a low pressure of between 10 and 20 kPa. The oxygen formed recombines partially with the metallic zinc of the anode, according to the reaction:Zn+½O2→ZnOZnO+2e−+H2O→Zn+2OH−
The zinc oxide is itself in equilibrium with the soluble form of the zinc in alkaline medium, zincate, according to the following simplified equation:ZnO+2OH−+H2O⇄Zn(OH)42−
In “sealed” mode, all of the gases formed must recombine in order to prevent an excessive increase in the internal pressure.
The operating principle of a sealed nickel-zinc battery such as that described above has its limitations for various reasons:                an excessive and uncontrolled charging that will lead to an excessive production of oxygen, the kinetics of reaction [1] prevailing over those of reactions [2] and [3],        as a result of the phenomenon described above, aggravated by a slower diffusion of the oxygen towards the negative electrode, the latter is completely charged, and a release of hydrogen then occurs:H2O+e−→OH−+½H2         the metallic zinc is thermodynamically unstable, and tends to corrode with the formation of hydrogen:Zn+2H2O→Zn(OH)2+H2        
The mode of management of the gases formed, oxygen and hydrogen, is a function of the design of the battery and its manufacture, the increase in internal pressure promoting the recombination of gas at the level of the electrode of opposite polarity to that where it forms, but being acceptable only within narrow limits in certain types of case.
Thus, an element of cylindrical shape with metallic case and cover supports pressures of more than 2000 kPa, while prismatic elements will accept maximum pressures of between 500 and 1000 kPa, as a function of the dimensions of the battery, the nature of the materials and the case/cover connection mode. On safety grounds, the covers of recombination batteries are fitted with valves. They are set to about 1500 kPa for cylindrical elements, and up to 200 kPa for prismatic formats.
The formation of hydrogen and its management constitute a particularly important aspect of the operation of a sealed nickel-zinc battery.
Various solutions have been proposed for limiting the pressure increase caused by the formation of hydrogen, including:                the use of catalysts based on silver, for example, which are incorporated in the positive electrode, and permit oxidation of the hydrogen during the charging according to the reaction:H2+2OH−→2H2O+2e−        the use of a third electrode, connected to the positive electrode and ensuring oxidation of the hydrogen;        the use of a catalytic structure consisting of carbon and platinum, which is deposited on a metallic collector or a carbon tissue responsible for ensuring the recombination of the hydrogen and the oxygen.        
Said various solutions are not however fully satisfactory, either because of a limited kinetics of oxidation of the hydrogen or because of a complex construction.
One of the limitations on the use of a catalytic structure for the recombination of hydrogen and oxygen is the heat management constraint of the system. The reaction between hydrogen and oxygen is highly exothermic, in fact, and can lead to a substantial increase in temperature, and to the formation of “hot spots” harmful to the efficient operation of the catalyst. It is therefore necessary to ensure a rapid removal of the calories produced during the recombination reaction.
Moreover, and this is another difficulty affecting the practical employment of catalytic structures, the water formed during the recombination of hydrogen and oxygen must not restrict access of the gases to the catalytic sites.
The aim of the present invention is to meet these various requirements: to this end, the inventors have developed catalytic structures using metal foams as supports, and an implementation suited to the intended use.
Said aim is achieved by a device for the catalytic recombination of gas for alkaline batteries with zinc anode, as well as an alkaline battery with zinc anode containing such a device, such as are defined in the claims.
The invention relates to a device for catalytic recombination of the gases formed during charging of an alkaline battery with zinc anode, characterised in that it is composed of a catalytic mass arranged in contact with a cross-linked cellular metal foam serving as catalyst support and heat dissipating structure, said catalytic mass being composed of a mixture of carbon black, including a platinum group metal and a hydrophobic binder, the whole being subjected to a heat treatment so as to cause the hydrophobic binder of said catalytic mass to be sintered.
Metal foams are widely used today in the alkaline battery industry as supports/collectors of electrodes. Said forms are made from a cross-linked cellular organic porous substrate, with open pores. The preferred substrates are polyurethane foams, commercial grade, exhibiting a good regularity of structure.
The methods of manufacture most widely used consist in rendering the organic foam conductive by an electronic conductive deposition, in then metallising it by electrochemical deposition(s), then removing any organic material by heat treatment, and finally in deoxidising and annealing the metal, the alloy or the deposited metals constituting the final cross-linked structure, which must retain its initial substantially or totally open porosity. These methods make it possible, in particular, to produce foams of nickel, of copper, or of alloys based on said metals, that are usable within the scope of the invention.
Within the scope of the present invention, as regards the recombination of the gases formed during the charging of the battery, the metal foam employed plays a dual role: it serves on the one hand as support for the catalyst of the reaction, and on the other it contributes to ensuring the removal of the calories produced on the recombination of the hydrogen and the oxygen.
As regards the heat dissipation, the latter is performed by radiation, convection and/or conduction. Said dissipation is all the better if the metal constituting the metal foam is itself a good conductor of heat. In order to optimise said characteristic, it is advantageous, in one of the embodiments of the invention, to use a copper foam, said metal being an excellent conductor of heat.
For such an embodiment, it will be advantageous to use foams of copper or alloys of copper, such as those that can be produced industrially under economic conditions according to the process described in French patent no. 2 737 507.